The processes of genetic recombination and recombinational repair are of fundamental importance in all organisms. Yet, the enzymes involved, their mechanism of action, and their regulation are not as yet understood, particularly in eukaryotes. It is the purpose of this research to initiate a coordinated biochemical and genetic investigation of these processes in a model eukaryote, Saccharomyces cerevisiae. In this organism, a regulatory system mediated by the genetic configuration at the mating-type locus has been described. Among its profound effects on the cell are the induction of meiosis (with its high frequency of recombination) and the stimulation of mitotic recombination and X-ray survival. Therefore, this study may ultimately lead to the elucidation of their control. Employing the regulatory system as a tool, recombination- and repair-specific enzymes will be partially purified and characterized, by comparing activities in extracts of cells "induced" for these enzymes with those found in extracts of control cultures. Using this approach, one activity (either a nuclease or phosphatase) has already been observed in meiotic cells (an "induced" state) that was not detected in mitotic control cells. The cellular roles of such activities will be studied by the screening of currently available mutants deficient in meiosis (particularly in meiotic recombination), mitotic recombination, and repair and of dominant mutants that have increased UV-induced mitotic recombination. These last strains will be isolated in the second part of the proposed research and are expected to be of two types, those with mutations at regulatory loci and those with alterations of structural genes for the enzymes involved. Such mutants will be crossed with the recessive mutants described above. It is hoped that these studies will provide new information about 1) the biochemical basis and control of recombination and repair and 2) the problem of genetic and biochemical regulation in eukaryotes in general.